The disk type brake is so high in stability at high speeds and under high loads as to be highly practical, not only for use with automobiles but also for motorcycles.
When a disk brake is employed for use on an autobicycle or motorcycle, the brake disk will be exposed on the outside of the wheel, as compared to a disk brake of an automobile. Therefore, the material of the disk should have various optimum characteristics associated with a brake disk, as well as having high corrosion resistance and favorable appearance. Therefore, a stainless steel material is used for such brake disks. In considering the characteristics of such stainless steel material used for brake disks, the brake performance depends on the friction coefficient of the surface of the material, i.e., the hardness and smoothness of the surface. If the hardness is too high, excessive slippage will result. If the hardness is low, excessive friction and wear will result. A squeaking sound of the brake may result from excessive hardness.
In the brake disk made of such stainless steel material and the process for producing the same, the performance of the brake disk is influenced by its hardness. In the prior art, the stainless steel plate material is generally first heated to 1050.degree. to 1150.degree. C. and quenched to obtain a material of HRC (a Rockwell hardness on the scale C) of about 50 to 53. Then it is annealed at about 650.degree. C. to reduce the hardness to HRC of 30 to 40, to obtain a brake disk of a required hardness. Particularly, the annular disk portion surface which is a brake pad sliding surface is treated to be of HRC of about 32 to 36, considering requirements for brake performance and subsequent machining.
Thus, as described hereinabove, the heat-treating process commonly presently employed to obtain brake disks is that of heating a material to be perfectly within an austenitic range, maintaining it in a heated state, quenching it to obtain a martensitic structure, and further annealing it to decompose the martensitic structure to obtain a carbide-sorbite structure.
In the above conventional means of obtaining brake disks, because the stainless steel material is once quenched and is then annealed, there will be increased distortions caused by the heat-treating steps. Also, even if a press-quenching means is adopted at the time of quenching to increase precision, a strain will be generated in the next annealing step, and poor precision in flatness and parallelism of the annular disk portion surface, which is a brake pad sliding surface, will be obtained. Further, in a disk in which an annular disk portion and wheel hub fitting portion are integrally shaped, there will be distortion generated on the whole, and the precision will be reduced. Therefore, such machining as cutting work will be required after shaping, and the production of brake disks will be troublesome and complicated. Also, mechanical working apparatus will be required and personnel will be required for the work. The producing steps will increase, there will be tool problems, etc., and the cost of the brake disk will of necessity be high. Further, because cutting work is required, it is necessry initially to make the material thicker. Thus, such steps will be disadvantageous to the production with respect to material and especially in shaping a thick material. Further, the annealing temperature range of 650.degree. to 750.degree. C. is so narrow that the above-mentioned required hardness will be difficult to obtain. Even in this respect, there are production problems such as in temperature control, etc.
In addition to the above, in the event that the above-mentioned brake disk is integrally press-shaped, a resilient returning action or springing back of the press-shaped material will occur in the bent shaped portion. Therefore, in particular, parallelism of the pad sliding surface will be difficult to obtain. Thus, the above-mentioned cutting work will be required for its correction. It is difficult to perfectly machine and correct a distorted annular disk surface, and, if the parallelism of the annular disk portion surface is not maintained at high precision, an uneven contact will be produced between it and the brake pad sliding surface. As a result, defects, such as uneven wear, will be produced on the annular disk portion surface, which is a braking surface of the brake disk. Also, uneven wear will be produced on the pad sliding surface of the brake calipers, and the braking efficiency will thus be reduced. Ultimately, the performance of the brake will be reduced, and the durability will also be influenced detrimentally.